A first cut at a function for edge length.  (Should be replaced by
a generic edge property visitor.)
changed name of object geting edge properties to EdgePropertyInspector as opposed to the always wrong and often misleading EdgeLengthVisitor.
interim checkin -- changed name of edge_length to edge_property and added
AttributeEdgePropertyInspector
Documentation and testing changes for edge property inspectors. (and some changes to the code so it matches the doco.
fixed typo in test
Merge pull request #99 from deinst/functional_edge_computation

Functional edge computation
Fix grammar nit in docs.